1. Field of the Invention
The present invention relates to a magnetic disk, and more particularly to a PERM (Pre-embossed Rigid Magnetic) disk from which data is reproduced with a magneto-resistance effect type head.
2. Description of the Related Art
As a magnetic recording medium often used for a computer, disc-like magnetic disks have been widely used on which data may be accessed at random. Of those magnetic disks, from a view of responsiveness, a sort of magnetic disk (so-called hard disk) has been selectively used which uses as its circular plates a hard material, such as a glass plate, a plastic plate, or an AI-alloy plate on which Ni--P is planted or alumite is treated.
This sort of magnetic disk has been requested to keep its recording density higher and higher. With the enhancement of the recording density, it has been requested that the media design and the recording and reproducing system are arranged to suit to a precode and encode system that can precisely convert an analog signal into the corresponding digital signal.
For recording data on this sort of magnetic disk, one or more magnetic disks for recording the data on each disk surface are combined with a ring head. The ring head utilizes an electromagnetic induction phenomenon. This ring head is mounted on a slider so that the head is run in the floating state from the magnetic disk surface. The distance between the head and the disk surface is minute and is kept by an air flow caused by the rotation of the disk.
This ring head, however, produces a far lower reproducing output as the track width is made narrower, which is an obstacle to securing a sufficient S/N ratio. It means that the increase of the density in the track width direction is restricted.
In order to overcome the disadvantageous restriction, a compound magnetic head composed of the ring head and the magneto-resistance effect head (simply called MR head) is now being used, where the ring head is responsible for recording data and the MR head is responsible for reproducing data. The MR head used in the compound magnetic head produces an output defined by the quantitive change of a magnetic flux at each circumferential unit length. Basically, as the track width is made narrower, the MR head keeps the output. Hence, the MR head is effective for a magnetic disk with a higher track density.
The foregoing description has been oriented to how the magnetic head overcomes the shortcoming entailed by a higher recording density. On the other hand, the magnetic disk has been devised for increasing the track density in various ways.
For example, for narrowing the track width, if the track width is made too narrow, a track suffers from crosstalk caused by a magnetic signal recorded on an adjacent track, which results in degrading the S/N ratio.
In order to suppress the crosstalk, there has been proposed a technique of forming on the plate surface convexo-concave patterns matching to the data tracks.
The convexo-concave patterns formed on the plate surface are directly reflected on the surface of a magnetic layer, so that the same convexo-concave patterns appear on the surface of the magnetic layer. For example, when a convex portion is matched to the data track, a concave portion is laid between these data tracks. This arrangement prompts the magnetic separation. Hence, if the track width is made relatively narrow, one data track may be protected from the adverse effect of the magnetic signal recorded on the adjacent data track. The arrangement, hence, offers an excellent off-track characteristic.
Further, as an application of this magnetic disk, a PERM disk has been developed (see MR 93-34, November, 1993 of Electronics and Communications Society). This PERM disk contains the data tracks and a servo signal formed as the convexo-concave patterns on the plate.
This PERM disk uses a plate molded from plastic, on which data such as the servo signal is pre-formatted. Hence, this PERM disk eliminates a troublesome work of writing the servo signal on a magnetic layer. This is advantageous in lowering the cost of the disk.
As described above, this sort of magnetic disk has been devised to cope with a higher recording density through the effect of the MR head or the PERM disk.
However, in order to enhance the recording density of the magnetic disk, it is necessary to develop the magnetic disk further.
That is, as mentioned above, the PERM disk has been arranged to allow the concave portion formed between the adjacent tracks to prompt the magnetic separation therebetween. But the hitherto known PERM disk cannot provide sufficient magnetic separation, so that as the distance between the adjacent tracks is made narrower, the crosstalk is still likely to appear. It means that the off-track characteristic may be improved further.
Moreover, the PERM disk or the MR head cannot provide a sufficient magnetic characteristic.
In reproducing a signal through the MR head, on the magnetic layer, it is desirous to keep a flux reversal transition width narrow, that is, keep a ratio of a product Mr.multidot..delta. to a coercive force Hc small, wherein Mr denotes a residual field and .delta. denotes a thickness of a magnetic layer. For example, about the coercive force Hc, for realizing a higher recording density than 1 Gbits/inch.sup.2, a higher coercive force than 2000 Oe (about 159 kA/m) is required.
Since the MR head dedicated to reproduction is used, priority is placed only on the recording characteristic when the ring head is designed. This design makes it possible to enhance the recording performance of the ring head and it is necessary to increase the coercive force of the recording medium.
About the coercive force, it has been reported that the metallic magnetic film enhances its coercive force if the plate is heated higher than 200.degree. C. when forming the metallic magnetic film. This technique, however, is improper to the PERM disk made of a plastic plate, because the plastic is thermally deformed.